Fresh sperm is viable for a relatively short period of time, typically only from about one to eight days. However, it is often advantageous to utilize sperm long after it has been collected, sometimes several months or even years later. Various methods, principally freezing, are employed to preserve sperm for these relatively longer periods of time. Freezing sperm permits, for example, a domestic animal breeder to maintain stocks of valuable sperm for use when necessary, enables the inexpensive transport of such stocks, and ultimately permits genetically superior males to inseminate a larger number of females. Beyond livestock, artificial insemination is also used in the human context for various medical and health reasons.
The preservation of sperm has proven to be quite troublesome. The survivability of viable cells using prior art freezing methods is often quite low. Freezing conditions are relatively harsh and thermal shock or other phenomena such as ice crystal formation often kill the sperm. Therefore, maximizing the viability of thawed cells has been the goal of many researchers.
The prior art discloses various methods for improving the survivability of frozen sperm. U.S. Pat. No. 4,007,087 (to Ericsson) discloses a sperm fractionation and storage method which claims to increase the percentage of motile sperm that survive frozen storage. Ericsson discloses a method whereby motile sperm are separated from non-motile, defective or dead sperm. The fraction containing the motile sperm is then frozen. Ericsson reports that his method increases the fertility of a sperm sample by enhancing the environmental (the ratio of total sperm to motile sperm) and viability (progressiveness of motility of the motile sperm) factors effecting the fertility of a sample, but his method does not improve the population (motile sperm count) factor which is possibly the most critical.
U.S. Pat. No. 3,791,384 (to Richter et al.) discloses a method for deep freezing and thawing boar sperm which includes inactivating the fresh sperm by means of an inactivating solution that includes dextrose, dihydrate of ethlenedinitrotetra-acetic acid, sodium citrate and sodium hydrogencarbonate. Richter reports that inactivation of the sperm gives them a greater power of resistance to freezing.
U.S. Pat. No. 4,429,542 (to Sakao et al.), U.S. Pat. No. 4,487,033 (to Sakao et al.), U.S. Pat. No. 3,893,308 (to Barkay et al.) and U.S. Pat. No. 4,480,682 (to Kameta et al.) all disclose different freezing methods which claim to improve the fertility of sperm samples. In all of these methods, the temperature of sperm in solution is lowered by various means which attempt to reduce the thermal shock and increase the survivability of the viable sperm and ova present. Most of these methods are, however, complex, cumbersome and expensive to utilize. Other freezing methods are also used including the "Sherman" method of rapid freezing in liquid nitrogen vapors (Sherman, J. K., Improved Methods of Preservation of Human Spermatozoa by Freezing and Freeze Drying, Fertil. Steril., 14:49-64 (1963), and the "Behrman-Sanada" method of gradual freezing (Behrman et al. Meterologous and Humologus Inseminations with Human Semen Frozen and Stored in a Liquid Nitrogen Refrigerator., Fertil. Steril. 17:457-466 (1966).
A disadvantage of the aforementioned methods resides in that low-temperature preservation of sperm is accompanied by the ice crystallization process. The ice crystallization process is retarded by the use of a cryoprotectant; however, the influence of the cryoprotectant on reducing ice crystallization is offset by the negative effects of the cryoprotectant on the ejaculate. Addition of a cryoprotectant typically results in injury to the cytoplasmic membrane of the sperm, because the addition leads to powerful osmotic shifts. The osmotic shifts cause partial denaturation of the protein molecules and disorientation of the cell organelles. In addition, if the sperm have prolonged exposure to a high concentration of cryoprotectant before freezing, there is also concern that the cryoprotectant will be toxic to the sperm.